Polybutylene terephthalate resins, which have excellent mechanical properties, electrical properties, thermal resistance, weather resistance, water resistance, chemical resistance, and solvent resistance, are widely used as engineering plastics for various uses such as automotive parts and electrical/electronic components.
Increasingly popular molded articles made of polybutylene terephthalate resins include thin platelike or thin-walled boxlike molded articles, for example, microswitch housings, small coil bobbins, thin connectors, and disk cartridge shutters, and the like; or intricately shaped molded articles. The molding of such a molded article with a resin having low fluidity is difficult due to molding defects likely resulting from insufficient filling of a mold for the molded article. In particular, an inorganic bulking agent such as, for example, glass fiber is often blended with a resin composition in order to improve mechanical properties depending on the field where it is used. However, such a resin composition has much lower fluidity, resulting in problems such as the development of warpage due to uneven flow of the resin in addition to probable molding defects. Therefore, a polybutylene terephthalate resin having improved fluidity has been desired.
As a method of improving the fluidity of a polybutylene terephthalate resin, Patent Document 1, for example, discloses a method including: blending polybutylene terephthalate species having different viscosities (number average molecular weights) in a predetermined proportion. Patent Document 1 also describes that a polybutylene terephthalate resin composition obtained in this way can improve the repeated-fatigue resistance of the resulting molded article, and also has high fluidity in the molten state. Nonetheless, even the above polybutylene terephthalate resin composition has an inferior resin extensibility and the like as compared with a case where highly viscous polybutylene terephthalate is used alone.
Further, Patent Document 2 discloses a method of improving fluidity while preventing leakage, the method including: blending a specific glycerin fatty acid ester with a polybutylene terephthalate resin. Specifically, a predetermined amount of a glycerin fatty acid ester having a hydroxyl value of 200 or more is blended as the specific glycerin fatty acid ester. This can improve fluidity while maintaining mechanical strength without causing problems such as metal contamination.
However, a molded article having a smaller, thinner, and also more complex shape with various functions has been demanded. In view of the above, a polybutylene terephthalate resin composition for producing these molded articles has been desired which has even further improved fluidity without impairing the properties of these molded articles.    Patent Document 1: Japanese Unexamined Patent Application, Publication No. H05-179114    Patent Document 2: PCT International Publication No. WO2009/050859